Field
The present invention relates to antimicrobial silanol quaternary ammonium compounds (SQACs) and a method for controlling the viscosity stability of aqueous dilutions of SQACs using naturally derived, renewable, volatile, phytochemical essential oils.
Related Art
A biocide is any substance that kills microorganisms such as bacteria, molds, algae, fungi or viruses. A biostatic is any substance that inhibits the growth of these organisms. The collective group is called antimicrobials. People have been utilizing antimicrobials, commonly called preservatives, since they first discovered a need to extend the useful life of their food as well as their possessions. Sea salt may have been the first antimicrobial used to preserve food. The mummification techniques employed by early Egyptians used to preserve the human and animal body used salts and a variety of resins. These preservatives were thought to possess magical powers, as well as the ability to install qualities of eternal life.
The existence of microorganisms in nature was discovered in the late 1600s with the invention of the microscope. As early as 1705, mercuric chloride was used to preserve ships' planking against shipworm. It was not until the 19th century discoveries by Pasteur, Gram and others that the causative agents of microbiological deterioration were understood, although use of antimicrobials in a cause and effect relationship with microorganisms is still less than a century old.
Certain silanol quaternary ammonium compounds (“SQACs”) possess bacteriostatic, fungistatic and algaestatic and/or bactericidal, fungicidal and algaecidal properties. For example, 3-(trimethoxysilyl)propyl octadecyldimethyl ammonium chloride is a commercial antimicrobial product marketed by Dow Corning as “BIOGUARD Q 9-5700”. A number of other organosilicon amines and salts also exhibit antimicrobial activity.
Reactive silanols are able to bond with a variety of target surfaces because they form a covalent bond with any surface containing oxygen, nitrogen or carbon in any form. For example, hydroxides or oxides on the surfaces of metals (including stainless steel) will form a durable bond. In addition, silanol groups will homopolymerize via a condensation mechanism to form a durable, 3 dimensional crosslinked polymer matrix. Reactive silanols are therefore able to bond with surfaces such as plastic, metal, fabric, tile, masonry, vinyl, wood, painted surfaces and human skin.
When silanols are modified with biocidal adjuncts in the form of alkyl quaternary ammonium groups, and the silanols are fixed onto a surface, the active biocidal sites become fixed too. The films created are extremely thin, between 15 nm and 180 nm, and therefore the original physical properties of the surface are little affected.
Silanols having biocidal adjuncts typically exhibit a mechanism of action whereby bacteria arriving on a treated surface will assimilate the biocidal adjunct's hydrocarbon, and the positively charged nitrogen atom will affect the electrical equilibrium of the cell. More specifically, the nitrogen atom disrupts the porin channels and/or outer protein layers, causing cell death.
The fixed nature of the biocide is important where toxicity, taint and other organoleptic aspects are of concern. This bactericidal surface treatment is not removed by normal cleaning procedures. In fact, it is important to maintain the normal cleaning regime in order to ‘refresh’ the biocidal surface. The thinness of the film enables application in areas where optical properties are important such as treatment of contact lenses. Silanols with biocidal adjuncts have been used in the treatment of bed sheets, hospital garments, curtains, floor and wall materials, air filtration systems, medical devices, bandages, surgical instruments and implants, and to prevent biofilm growth on catheters, stints, contact lenses and endrotracheal tubes.
Based toxicity information, the EPA concluded that there are no endpoints of concern for repeated oral or dermal exposure to the trimethoxysilyl quats. This conclusion was based on low toxicity observed in acute, subchronic and developmental studies conducted with the trimethoxysilyl quat compounds. They further concluded that there are no concerns for carcinogenicity for the trimethoxysilyl quats based on the results of the mutagenicity studies and the lack of any systemic toxicity in the toxicity database.
Based on hydrolysis data, the EPA has concluded that trimethoxysilyl quats are soluble but not stable in water. They stated that due the instability of the compounds and their formation of an insoluble silane degradate, that the trimethoxysilyl quats are not expected to contaminate surface or ground water due to rapid degradation by hydrolysis.
While aqueous SQACs have a tremendous amount of potential as antimicrobials, there are significant shortcomings. They are very unstable and have a short shelf life. For example, premature sedimentation of polysilsesquioxane-type polymers occurs in even low aqueous concentrations. Also, premature polymerization causes unwanted solution viscosity, thereby complicating conventional coating methods.
A variety of strategies have been employed in order to extend the storage life of aqueous SQACs. Examples include introducing surfactant additives, to coordinate the free silanol ends with stabilizers such as simple sugars and other multiple hydroxyl group molecules; coordinating and associating said quaternary organosilane hydrolysates with hydrophilic polymers; incorporating non-aqueous solvents such as the toxic methanol and methyl or butyl cellosolve, using alternative aqueous/organic systems, and combinations thereof. In some cases, pH adjustments have been used to maximize the benefits imparted by a stabilizer. These strategies all have shortcomings including undesirably creating a hydrophilic toxicity and cost.
It is desirable that the aqueous medium contains additives and components that eliminate or decrease the premature homopolymerization of the hydrolyzed silanol groups, thereby increasing storage stability. It is desirable that the aqueous medium contains additives and components that eliminate or decrease unwanted increases in viscosity arising from premature homopolymerization. It is desirable that the aqueous medium contains additives and components that eliminate or decrease unwanted precipitation arising from premature homopolymerization. It is also desirable that the aqueous medium contains additives and components that impart a pleasant scent, maintain solution clarity, improve performance of the underlying SQAC, and protect the composition against aqueous mold growth. It is desirable that the additives will evaporate completely during the coating and curing operation, thus allowing the generation of a high degree of homopolymer crosslinking of the silanol groups, thereby providing a highly water and solvent insoluble coating on the substrate. It is desirable that the additives are not hydrophilic and that they do no alter the cationic charge density of the SQAC. It is also desirable that the additives and components are environmentally friendly.
In short, it is desirable to employ SQAC stabilizers having the following characteristics: 1) Low Toxicity; 2) Low Flammability; 3) Excellent stabilization of aqueous SQACs; 4) Pleasant scent; 5) Volatility (little or no incorporation of the stabilizer into the cured film); 6) Antimicrobial Activity; and 7) Obtained from a renewable resource.
Unexpectedly, a unique group of volatile essential oil stabilizers has been discovered that satisfies all 7 of the above attributes needed to correct the shortcomings of previous inventions of this type. These unique stabilizers are certain naturally derived, renewable, volatile, phytochemical essential oils (essential means “having an essence”) that have been proven to possess low toxicity, low flammability, excellent stabilization of aqueous SQACs, pleasant scent, good volatility, and demonstrate antimicrobial activity of their own.